In an optical fiber amplifier based on Raman amplification, it is possible to realize a high-gain optical fiber amplifier by using a high power pumping source employing a plurality of semiconductor laser devices. Since the Raman amplification of a signal light takes place when the polarization of the signal light and the polarization of a pump light are in correct matching, it is necessary to reduce the effect of polarization mismatching between the signal light and the pump light. To achieve this, degree of polarization (DOP) is reduced by eliminating the polarization of the pump light (i.e., depolarization of the pump light).
FIG. 26 is a block diagram of one example of a conventional Raman amplifier used in a WDM communication system. In FIG. 26, semiconductor laser modules 182a to 182d which include Fabry-Perot type semiconductor light-emitting elements 180a to 180d and fiber gratings 181a to 181d in corresponding pairs, output to polarization-combining couplers 161a and 161b laser beams to constitute the pump light. The semiconductor laser modules 182a and 182b output laser beams of the same wavelength. However, the polarization-combining coupler 161a combines the laser beams, aligned to have different polarization directions. Similarly, the semiconductor laser modules 182c and 182d output laser beams of the same wavelength. However, the polarization-combining coupler 161b combines the laser beams, aligned to have different polarization directions. The polarization-combining couplers 161a and 161b output the polarization-combined laser beams to the WDM coupler 162. The laser beams output from the polarization-combining couplers 161a and 161b have different wavelengths.
The WDM coupler 162 couples the laser beams output from the polarization-combining couplers 161a and 161b and outputs the coupled laser beam to an amplification fiber 164 as the pump light through an isolator 160 and a WDM coupler 165. The signal light to be amplified is input from a signal light input fiber 169 through an isolator 163 to the amplification fiber 164 to which the pump light is input, where the signal light is coupled with the pump light and Raman amplified.
The process of manufacturing the optical fiber amplifier will be complicated if the laser beams to be polarization-combined are emitted from stripe structures of different semiconductor elements. The size of the optical fiber amplifier also needs to be scaled up. Therefore, in order to solve these problems, a method of fabricating a Raman amplifier by using a semiconductor laser device that has two stripes on a single semiconductor substrate is proposed in Japanese Patent Laid-Open Publication No. 2001-402819. In this instance, it is possible to simplify the manufacturing process and to downsize the semiconductor laser device itself since a plurality of stripes is fabricated on the single substrate.
However, in the semiconductor laser device that has two stripes (hereinafter “W stripe laser device”) as disclosed in the above Japanese Patent Laid-Open Publication, the two stripes arranged in parallel to each other are disposed extremely close to each other, with a spacing not more than 100 μm, or about 40 μm for instance, with a resonator formed by the common cleavage surface. The two stripes have almost the same physical structure and, hence, their resonator lengths are almost identical. Besides, due to proximity of the two stripes, the temperatures of their active layers are almost the same. Consequently, oscillation longitudinal mode wavelengths of the emitted laser beams, as well as a spacing of a plurality of oscillation longitudinal modes, are likely to coincide between the two stripes. If the oscillation longitudinal modes of the laser beams emitted from the different stripes overlaps, it is not possible any longer to reduce the DOP by polarization-combining the laser beams. This problem, though more conspicuous in a W stripe laser device, may occur even if the two stripes are disposed on different substrates. The phenomenon may be considered to occur because the overlapping of the oscillation longitudinal modes of the two laser beams that are polarization-combined may reduce the fluctuation of phase difference of the two oscillation longitudinal modes being combined, especially when the line width of the oscillation longitudinal modes are narrow, giving rise to a particular polarization state corresponding to the phase difference of the two modes in the combined laser beam.
Another problem arises due to overlapping of oscillation longitudinal modes of the laser beams that are polarization-combined. Normally, immediately after polarization-combining, the polarization components of the laser beams emitted from the two stripes do not interfere with each other. However, when RIN (relative intensity noise) for a laser beam propagated over a long distance is measured, a peak corresponding to a beat noise is observed in the vicinity of 11 GHz, as shown in FIG. 27, as a result of a mixing between the orthogonal polarization components during the long-distance transmission over the optical fiber. Since the Raman amplification in particular is a nonlinear process that takes place in an extremely short timescale, a noise that would develop as shown in FIG. 27 due to the beat noise when using the W stripe laser device as the pump light source, would be translated into the signal noise that would hamper the signal transmission.
Therefore, it is an object of the present invention to realize a semiconductor laser device and a semiconductor laser module which are suitable for an pump light source such as a Raman amplifier, and in which the degree of polarization is minimal and the beat noise, owing to long-distance transmission, does not occur, and an optical fiber amplifier using the semiconductor laser module, which enables a stable high-gain amplification independently of the polarization direction of the signal light.